Critical Downstream Locations

    A critical downstream location is a place that is prone to flooding. The criteria of establishing a Critical Downstream Location is given in the City of Columbia Code of Ordinances, Section 12A-5 Definitions:

    Critical downstream location – A location within the drainage reach downstream of the subject site, consisting of a channel section, drainage swale, bridge, box culvert, storm sewer, or other conveyance facility or structure having a conveyance capacity which would be exceeded by stormwater runoff from a 10-year frequency, 24-hour duration storm under existing land use conditions; or an existing structure or building located downstream of the subject site which has its lowest floor elevation less than one (1) foot above the maximum elevation in an adjacent channel attained by the 100-year frequency, 24-hour duration storm, assuming existing land use conditions with the proposed ultimate development of the subject site in place. The conveyance capacity of a structure operating under inlet control conditions shall be determined with a maximum headwater to diameter ration (HW/D) of 1.25 or with a headwater elevation equal to the top of curb, whichever is less.

    Redevelopment projects which take place upstream of these locations are required to have more stormwater management in order to help address the flooding problem, but the location has to be mapped when the project comes forward in order for the stricter stormwater management requirements to be applied.

    The Critical Downstream Locations Map is updated twice every year on:

    January 1 and July 1, starting with July 1, 2013.

    The map is available in several forms, including a poster, a map book, and an interactive web map. No map is more current than the other. Poster and map book versions are large .pdf files and may take time to download. The web map is designed for use on desktop or laptop computers with Windows or Apple operating system.

    Poster Map Book Web Map

    Updated 1-1-2016

    2016-01-01 Poster

    Updated 1-1-2016

    2016-01-01 Map Book

    Updated 1-1-2016

    Web Map

    No changes were made to the data or maps in the update cycles ending June 30, 2014, December 21, 2014, June 20, 2016, or December 21, 2016.

    The data of January 1, 2016 is the current map.

    The next update cycle ends June 21, 2017.

    Those who need to know what will be on the next map revision can call Public Works at 874-7250 and ask to speak to someone in the stormwater utility about the Critical Downstream Locations map.